Java碎碎念之Java比较器(comparable接口与comparator接口)

A:Comparable接口的使用举例:(自然排序)
1,像String,包装类等实现了Comparable接口,重写了comparable()方法,给出了比较两个对象大小的方式.
2,像String,包装类等重写comparableTo方法以后,进行了从小到大的排列.

  String[] str={"f","e","d","c","b","a"};
        Arrays.sort(str);
        System.out.println(Arrays.toString(str));

[a, b, c, d, e, f]

3,重写comparableTo(obj)的规则:
如果当前对象this大于形参对象obj,则返回正整数,
如果当前对象小于新参对象obj,则返回负整数,
如果当前对象this等于形参对象obj,则返回零.

package com.compareabletest;

import java.util.Arrays;


public class StringTest {
    public static void main(String[] args) {
        Goods[] goods =new Goods[4];
        goods[0] = new Goods("jane", 12);
        goods[1] = new Goods("jane", 1);
        goods[2] = new Goods("jane", 2);
        goods[3] = new Goods("jane", 22);
        Arrays.sort(goods);
       System.out.println(Arrays.toString(goods));
    }
}

class Goods implements Comparable {
    String name;
    int age;

    public Goods() {
    }

    @Override
    public String toString() {
        return "Goods{" +
                "name='" + name + ''' +
                ", age=" + age +
                '}';
    }

    public Goods(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public int compareTo(Object o) {
        if (o instanceof Goods) {
            Goods goods = (Goods) o;
            if (this.age > goods.age) {
                return 1;
            } else if (this.age < goods.age) {
                return -1;
            } else return 0;
        } else {
            return -1;
        }
    }
}

[Goods{name='jane', age=1}, Goods{name='jane', age=2}, Goods{name='jane', age=12}, Goods{name='jane', age=22}]

B:Comparetor接口的使用:定制排序
1,背景:
当元素的类型没有实现java.lang.Comparable接口而又不方便修改代码,或者实现了java.lang.Comparable接口的排序规则不适合当前的操作,那么可以考虑使用Comparator的对象来排序.
2,重写compare(Object o1,Object o2)方法,比较o1,o2的大小;正数,o1大;负数o2大;0一样大.
实例代码:①将String数组逆序排列

package com.compareabletest;

import java.util.Arrays;
import java.util.Comparator;


public class StringComparableTest {
    public static void main(String[] args) {
        String[] strs = {"a", "b", "c", "d"};
        Arrays.sort(strs, new Comparator() {
            @Override
            public int compare(String o1, String o2) {
                return -o1.compareTo(o2);
            }
        });
        System.out.println(Arrays.toString(strs));
    }
}

[a, b, c, d]

②自定义类

package com.compareabletest;

import java.util.Arrays;
import java.util.Comparator;


public class GoodsComparableTest {
    public static void main(String[] args) {
        Goods[] goods = new Goods[4];
        goods[0] = new Goods("a", 12);
        goods[1] = new Goods("c", 1);
        goods[2] = new Goods("b", 2);
        goods[3] = new Goods("a", 22);
        Arrays.sort(goods, new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                if (o1 instanceof Goods && o2 instanceof Goods) {
                    Goods g1 = (Goods) o1;
                    Goods g2 = (Goods) o2;
                    if (g1.getName().equals(g2.getName())) {
                        return -Integer.compare(g1.age, g2.getAge());
                    } else return g1.getName().compareTo(g2.getName());
                } else throw new RuntimeException("类型不匹配!");
            }
        });
        System.out.println(Arrays.toString(goods));
    }
}

class Goods implements Comparable {
    String name;
    int age;

    public Goods() {
    }

    @Override
    public int compareTo(Object o) {
        if (o instanceof Goods) {
            Goods goods = (Goods) o;
            if (this.age > goods.age) {
                return 1;
            } else if (this.age < goods.age) {
                return -1;
            } else return 0;
        } else {
            return -1;
        }
    }

    public Goods(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Goods{" +
                "name='" + name + ''' +
                ", age=" + age +
                '}';
    }

    public int getAge() {
        return age;
    }
}

[Goods{name='a', age=22}, Goods{name='a', age=12}, Goods{name='b', age=2}, Goods{name='c', age=1}]

C:Comparable接口与Comparator接口的对比
Comparable接口一旦一定,保证Comparable接口实现类的对象在任何位置都可以比较大小
Comparator接口属于临时性的比较.